The SIPROTEC 7SS85 is the busbar differential protection relay of the Siemens SIPROTEC 5 family, and the 7SS52 is its SIPROTEC 4 predecessor, a distributed system with a central unit and bay units. Both protect high- and extra-high-voltage busbars with 87B differential and 50BF breaker failure. Siemens presents the 7SS85 as the replacement for the 7SS52, with the same algorithms.
What it is and where it is used
The SIPROTEC 7SS85 relay is the busbar differential protection of the Siemens SIPROTEC 5 family, and the SIPROTEC 7SS52 is its predecessor in the SIPROTEC 4 generation, still widely installed. Both protect the busbars of high- and extra-high-voltage substations, and also of medium-voltage ones when the importance of the busbar justifies it. They compare the currents of all the connected bays. If the sum does not balance, they trip only the breakers of the faulted busbar.
They are used in double busbar, breaker-and-a-half, main and transfer bus and sectionalized busbar arrangements, where the busbar image (which bay is on which busbar, according to the disconnectors) is part of the protection. For the principle see busbar differential protection 87B and busbar protection schemes.
Variants: 7SS85, 7SS52 and bay units
| Model | Application | Main difference |
|---|---|---|
| SIPROTEC 7SS85 | HV and EHV busbars, new projects and retrofit | SIPROTEC 5; centralized, distributed or hybrid; process bus |
| SIPROTEC 7SS52 (central unit) | HV and EHV busbars, legacy | SIPROTEC 4, distributed system |
| 7SS52 bay units (for example, 7SS5231…) | One per bay | Measure current and status and execute the bay trips |
| 7SS85 bay units | Distributed or hybrid architecture | SIPROTEC 5 devices or merging units depending on the design |
Main functions
| Function | ANSI | Comment |
|---|---|---|
| Busbar differential with stabilization | 87B | Main protection per zone (87B guide) |
| Check zone | 87B | Additional criterion independent of the disconnectors |
| Breaker failure | 50BF | Busbar trip if a breaker does not open (guide) |
| Dead zone and end fault protection | 50 | Faults between the breaker and the current transformer (guide) |
| Current and current transformer supervision | — | Blocks on open circuits or saturation |
| Dynamic busbar image | — | Assignment of bays according to the disconnectors |
The exact set of functions and the number of bays and zones depend on the variant and on what was ordered.
7SS85 performance data
The Siemens product profile (2022 edition, in English) declares these values:
| Item | Declared value |
|---|---|
| Tripping, centralized protection | 3 ms (HS-type relay) / 7 ms (F type) |
| Tripping, distributed protection | 8 ms (HS type) / 12 ms (F type) |
| CT saturation-free time to recognize an internal or external fault | 2 ms |
| Algorithms | Those of the 7SS52, proven since 1989 |
| Three interacting measurement methods | Minimum tripping time and maximum stability with high short-circuit currents |
Other features that Siemens lists in that profile:
- Operating characteristic with freely adjustable characteristic sections and an additional curve of higher sensitivity for low-current faults, for example in resistance-grounded systems.
- Stabilization with circulating currents for external faults, even with CT saturation; closed-core or linearized CTs can be used, and different CT ratios are adapted by parameterization.
- Breaker failure for one or three poles using the integrated disconnector image to trip all the breakers of the affected section; if the bus coupler breaker fails, it trips the adjacent busbar. See also 50BF operation without breaker failure.
- Dead zone protection between breaker and CT on feeders and couplers, and coupler differential for couplers with two CTs.
- Direct tripping of zones by external signals and release of the trip by additional external signals, even per phase.
- Cross-stabilization against overfunction caused by the influence of the transformer on the secondary side.
- Extensive supervision of CT circuits, measurement acquisition and trip circuits, to avoid overfunction and underfunction and reduce routine testing.
- Special operating states: bay out of service, blocking of disconnector and breaker acquisition and blocking of zones.
- Any feeder protection function can be implemented with a modular SIPROTEC 5 device as a bay unit in the distributed architecture.
Siemens also declares cybersecurity with role-based access control, security event logging, signed firmware and IEEE 802.1X authentication, and compliance with the NERC CIP requirements and the BDEW white paper. For IEC 61850 it mentions compatibility with merging units according to edition 2.1, and an integrated IEEE 1588 PTP grandmaster clock to synchronize the sampled values. The product page indicates support for configurations from single busbar to five-way, breaker-and-a-half, double breaker, withdrawable switchgear, combinations, H configuration and ring busbars. See IEEE 1686 and IED security.
7SS52 capacity and features
A Siemens technical document on the 7SS52, which we consulted in a third-party copy, gives the system capacity data:
- Up to 48 bays, 16 bus couplers, 24 sectionalizing disconnectors and 12 busbar sections, in single, double, triple or quadruple busbar systems, with or without a transfer bus.
- Tripping in under 15 ms and a saturation-free time of at least 2 ms. It trips only when the three detection modules (two measuring processors and the check zone) recognize a busbar fault.
- Bay units that measure the three currents with a 1 A or 5 A input (no auxiliary CTs), receive disconnector status through configurable binary inputs and execute the trips of their feeder. The 7SS523 variant uses the standard 7XP20 housing, flush or surface mounted.
- Central unit in a 19-inch rack, connected to the bay units by optical fiber in a star, with serial ports for a PC and for the substation control system.
- Breaker failure with five operating modes per bay, separate settings for busbar fault and line fault, two stages (trip repetition and then busbar trip) and a low-current mode with auxiliary contacts.
- Communications: IEC 60870-5-103 optical interface and, with the EN100 module (firmware V4.6), IEC 61850 electrical interface.
Siemens now classifies the 7SS52 as a phase-out product on its product page. In a replacement project, the real problem is rarely the relay: it is the disconnectors that feed the busbar image, the current wiring of each bay and the testing of the whole. See 87B trip when operating disconnectors and disconnector replica for the 87B. To go further: case of the 87B that tripped the bus during a bay test.
Communications and integration
The 7SS85 shares the SIPROTEC 5 platform: IEC 61850 (with GOOSE and, in process bus architectures, merging unit sampled values), PRP/HSR redundancy and time synchronization; it is configured with DIGSI 5. The 7SS52 uses the SIPROTEC 4 generation and is configured with DIGSI 4. See Siemens DIGSI software, merging unit and PRP and HSR.
Frequently asked technical questions
- 7SS52 and 7SS85 manuals: Siemens publishes the manuals in its SIPROTEC library; the manual version must match the firmware version.
- Busbar trip because of a wrongly reported disconnector: the busbar image depends on the auxiliary contacts of the disconnectors; a false status can cause a trip or a block. That is why the check zone exists.
- Testing the busbar protection: it requires injecting all the bays of the zone, simulating the disconnector positions and testing breaker failure without tripping busbars in service. The test plan follows the local code and the utility’s requirements.
If your device is no longer manufactured
If your substation has a 7SS52, Siemens proposes the 7SS85 as the successor, with the same algorithms and prepared for retrofit. The migration has these steps: survey the current configuration and the busbar image, define the new architecture (centralized, distributed or hybrid), adapt wiring and panels, redo the IEC 61850 integration and test in stages. We configure the 7SS85 remotely, design the replacement and perform the tests.
How we help
At Tiza Group we configure and review SIPROTEC relays remotely, worldwide, including busbar protection, starting from the approved settings and leaving a backup before and after the change. See remote configuration of Siemens SIPROTEC relays. If the busbar tripped and it is not clear why, send us the event for trip analysis. For the retrofit of a 7SS52, we design the change in secondary engineering. Write to us through the contact page.
Related guides
- Busbar differential protection 87B
- Busbar protection schemes
- Breaker failure protection 50BF
- Busbar protection by reverse interlocking
- Siemens DIGSI software
- Case: the 87B tripped the bus while testing another bay
- 50BF Operation Without Breaker Failure: Causes
References
- IEC 60255 series, measuring relays and protection equipment.
- IEC 61850, communication in substations; IEC 62439-3, PRP and HSR.
- Siemens, SIPROTEC 7SS85 profile (English, edition 08.2022), 7SS85 product pages (German and English) and 7SS52, and technical workshops «SIPROTEC 7SS85 – busbar protection» (7SS52 retrofit).
- IEEE C37.234, guide for protective relay applications to power system buses (2021 edition): effect of the availability of breakers, CTs and disconnectors on the choice of scheme, breaker failure and security against corrupted input signals.
Frequently asked questions
What is the difference between the 7SS52 and the 7SS85?
The 7SS52 is the busbar protection system of the SIPROTEC 4 generation: distributed, with a central unit and bay units connected by fiber. The 7SS85 is the SIPROTEC 5 one and supports centralized, distributed or hybrid architecture, plus process bus with merging units. Siemens presents it as the replacement for the 7SS52, with the same protection algorithms.
What is the 7SS5231-5CA01-0AA1?
It is an order code for a device of the 7SS52 family. The full code indicates which unit it is within the distributed system and its hardware, power supply and communication options. To know what an installed device has, you have to read the nameplate and the full code, not just the prefix.
Can a 7SS52 be migrated to a 7SS85 without replacing everything?
Siemens states that the 7SS85 is prepared for retrofit of 7SS52 installations. In practice the migration involves reviewing the current wiring and disconnector status wiring, translating the busbar image configuration, redoing the integration and testing the whole scheme with the substation in service or with planned outages.